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unrelated.py: examples with ``super()`` in a sibling class.
``U`` is unrelated (does not subclass ``Root``)
Calling ``ping`` on an instance of ``U`` fails::
# tag::UNRELATED_DEMO_1[]
    >>> u = U()
    >>> u.ping()
    Traceback (most recent call last):
    AttributeError: 'super' object has no attribute 'ping'
# end::UNRELATED_DEMO_1[]
But if ``U`` is part of a cooperative arrangement of base classes,
its ``ping`` method works::
# tag::UNRELATED_DEMO_2[]
    >>> leaf2 = LeafUA()
    >>> leaf2.ping()
    <instance of LeafUA>.ping() in LeafUA
    <instance of LeafUA>.ping() in U
    <instance of LeafUA>.ping() in A
    <instance of LeafUA>.ping() in Root
    >>> LeafUA.__mro__  # doctest:+NORMALIZE_WHITESPACE
    (<class 'diamond2.LeafUA'>, <class 'diamond2.U'>,
     <class 'diamond.A'>, <class 'diamond.Root'>, <class 'object'>)
# end::UNRELATED_DEMO_2[]
Here ``U.ping`` is never called because ``Root.ping`` does not call ``super``.
    >>> o6 = LeafAU()
    >>> o6.ping()
    <instance of LeafAU>.ping() in LeafAU
    <instance of LeafAU>.ping() in A
    <instance of LeafAU>.ping() in Root
    >>> LeafAU.__mro__  # doctest:+NORMALIZE_WHITESPACE
    (<class 'diamond2.LeafAU'>, <class 'diamond.A'>, <class 'diamond.Root'>,
     <class 'diamond2.U'>, <class 'object'>)
# tag::DIAMOND_CLASSES[]
from diamond import A  # <1>
class U():  # <2>
    def ping(self):
        print(f'{self}.ping() in U')
        super().ping()  # <3>
class LeafUA(U, A):  # <4>
    def ping(self):
        print(f'{self}.ping() in LeafUA')
        super().ping()
# end::DIAMOND_CLASSES[]
class LeafAU(A, U):
    def ping(self):
        print(f'{self}.ping() in LeafAU')
        super().ping()

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